1. Field of the Invention
The present invention relates to drive control apparatuses for solid-state light-emitting devices and, more particularly, to semiconductor laser apparatuses used for highly advanced electronic equipments such as an optical data communication system, an optical information record/reproduction apparatus, and an optical hard copy generator.
2. Description of the Related Art
Along with increasing demands for higher performance and higher reliability of the recent electronic equipments, strong demand has arisen for developing a semiconductor laser serving as a light source for emitting a light beam representing analog or digital information. Contradictory technical problems, i.e., an increase in data capacity vs. a high data transfer speed, lie in these electronic equipments. In order to solve these problems, an apparatus for driving and controlling a semiconductor laser to maintain an actual output light amount of the semiconductor laser at a stable optimal level has much room for improvement in control performance such as precision of intensity modulation.
Semiconductor lasers have many advantages: intensity modulation can be directly modulated; the size is small; power consumption is low; and emission efficiency of the laser beam is excellent. Therefore, semiconductor lasers are currently used in a variety of applications as in an optical data communication system, an optical information record/reproduction apparatus, and an optical hard copy generator. Presently available semiconductor laser apparatuses, however, are susceptible to abrupt variations in output light amount and deterioration over time.
The variations in output light amount of the laser beam are mainly caused by:
(1) changes in differential quantum efficiency caused by changes in ambient temperature and deterioration over time;
(2) changes in oscillation threshold value adversely affected by temperature changes and/or its own reflected light (return light) component; and
(3) noise caused by the return component (this noise is known as "mode hopping noise"). In order to suppress or inhibit the variations in output light amount of the semiconductor laser, the semiconductor laser apparatus must have higher laser beam intensity modulation precision to eliminate various factors for varying the output light amount.
An existing semiconductor laser control apparatus is conventionally provided with a photodetector for monitoring laser output light, such as a photodiode. A detection signal from the monitoring photodetector is converted by using a resistor and the like into a monitor voltage signal representing an actual output light amount. A monitor voltage signal is supplied to an operational amplifier. The operational amplifier compares a reference voltage signal representing a desirable output light amount with the monitor voltage signal. If a difference between these two input signals is detected, an error signal corresponding to this difference is generated by the operational amplifier. The error signal is fed back to a semiconductor laser driver, so that the output light amount of the semiconductor laser is corrected to come close to the reference level.
With such an arrangement, however, a controllable frequency range of the control apparatus is kept low, and precision of laser beam intensity modulation is not necessarily satisfactory. In the near future, the control apparatus may not be able to satisfactorily cope with applications to large-capacity, high-speed data processing systems due to the following three reasons. First, the presence of a junction capacitance of the monitoring photodetector will limit expansion of the control frequency range. Second, low-pass filter (LPF) characteristics of the monitoring photodetector also interfere with expansion of the control frequency range. Third, an increase in transmission time of a feedback signal necessarily generated within a control loop of a semiconductor laser may cause a decrease in phase margin of the control system.
In "High-Performance Rewritable Optical Data Recording Technique", Mitsubishi Denki Gijitsu Koho, Vol. 7, pp. 26-29, 1988, a semiconductor control circuit applied to an optical disk system so as to perform two different laser oscillation stabilization operations is disclosed. This control circuit employs a narrow control frequency range feedback (closed loop) control system for compensating a threshold value of a semiconductor laser and employs an open loop control system for variations in differential quantum efficiency. According to these two types of laser oscillation stabilization control schemes, since independent circuit arrangements are required for the respective control systems in principle, the arrangement of the apparatus is undesirably complicated as a whole. More importantly, in an optical data recording mode, a housing of a head unit which stores the semiconductor laser is heated due to a temperature change caused by variations in differential quantum efficiency. As a result, the precision of light intensity modulation is degraded. This degradation causes decisive degradation of performance in laser stabilization control. Therefore, this prior art is expected to be not capable of coping with future increases in capacity and speed of optical disk systems.